Apparently, the biggest problem is power consumption, because Logitech made it with four levels of power drain;
when in use, the red LED in the bottom blinks 1500 times a second, providing the mouse with a resolution of 800 dpi, the highest yet for any optical mouse according to Logitech. The second you stop moving the mouse around, it switches to the second level, blinking 100 times a second.
If you leave it alone for a while, it steps down to 10, then 2 times a second.

Because of the high power drain, the mouse uses two AA batteries opposed to the AAA batteries Logitech's normal cordless mice use.
This secures the approximate three months of operation that the normal cordless mouse has, but it makes the mouse slightly larger.

And now, ladies and germs, the moment you've all been waiting for:
The review

The design is really comfortable, and the added thumb button makes it genuinely cool, upping the total number of buttons to four, counting the wheel.
The resolution is terrific, and there's not a hint of the sometimes jerky movement you'd get with a normal cordless mouse or, indeed, a vanillaball-and-chain mouse.
There's hardly any resistance when you move it around, and it's not much heavier than regular mice.

All in all, it's incredible for a first-of-its-kind product, and you'll wonder what you ever did without it. Get one. Now.